In a V type or horizontal opposition engine, in order to make effective use of space and to make the engine compact, cam shaft drive sprockets are normally placed at either of the two ends of the crank shaft, and the respective cam shafts are driven by means of the said sprockets and a cam chain.
Where the engine has a number of valves on the air intake side, in order to facilitate maintenance, if the spark plugs are to be installed from the cam chain side, they must of necessity be located on the exhaust valve side.
Accordingly, as shown in FIGS. 8 and 9 illustrating the prior art, in the case of a V type engine, for example, the cam chains 104, 105 which go around cam shaft drive sprockets 102, 103 at the two ends of crank shaft 101, etc., are respectively wound around secondary sprocket 107 of the front cylinder head 106 and secondary sprocket 109 of the rear cylinder head 108, so as to drive their respective cam shafts 111 and 112. With the front and rear cam shaft drive sprockets 102 and 103 fitted in reciprocally corresponding positions, with the respective cylinder heads 106 and 108 having their cam chain chamber 113 and 114 placed in mutually reversed positions, and with spark plugs 121 and 122 installed from the cam chain chamber side, in order to prevent interference involving the taut sides 104a and 105a and the slack sides 104b and 105b of the cam chains 104 and 105, respectively, the left to right angles of deviation from the central lines 106a and 108a for the respective cylinder heads 106 and 108 must be .theta.1 and .theta.2. Accordingly, the respective cylinder heads 106 and 108 must be manufactured separately, with a number of separate component parts, making it difficult to achieve cost reductions.
If the installation angles .theta.1 and .theta.2 are made equal for plugs 121 and 122, and if one attempts to use the two cylinder heads 106 and 108 together, plugs 121 and 122 will interfere with cam chains 104 and 105, respectively, so that it becomes necessary to enlarge the secondary sprockets 107 and 109. This in turn requires larger cylinder heads 106 and 108, precluding the possibility of making the engine more compact.